I'm on day 18 and I experienced a similar thing although earlier on in my cleanse. It lasted about 3 days and I had moments of real discomfort. Found it hard to get comfortable lying down too. The pain also spread to my upper back, but only for a short while.
According to Stanley Burroughs, all back pain is connected to constipation (his theory.) I agree with the earlier post suggesting the salt water flush. Tough it's not fun to do, it does help clear you out and therefore help to ease the pain. I also soaked in a hot tub with dead Sea Salt s while burning some incense and it was very soothing.
Good luck and keep up the good work! Peace